This invention generally relates to air filters. More particularly, this invention relates to air filters having varying porosity and an electrostatic charge applied to the filter material.
Air filters are used in a variety of applications. One particular use includes air handlers for heating and cooling systems within buildings. Air filters typically are placed within an air handler to filter out dust particles from the air that are present within the xe2x80x9creturnxe2x80x9d flow from the building, which is conditioned (i.e. heated or cooled) before being returned to the building in a conventional manner.
There are several competing factors that influence the design of an air filter. Utilizing very low porosity filter material provides the ability to filter out particles from the air down to very minute sizes. Such material, however, often becomes relatively quickly congested or plugged by the particles collected. Because the porosity is so low, all particle sizes above that set for the particular material are gathered by the material and tend to clog the material. Accordingly, low porosity materials tend to have a limited life and cause pressure drop in the flow of air.
Other materials having higher porosity tend to last longer and not have the associated pressure drop, however, the ability to filter out minute particles is compromised.
One advancement in the filter art has been to apply an electrostatic field to a filter material to enhance the ability of the material to collect particles of different sizes. Such arrangements are shown in U.S. Pat. No. 5,549,735 and U.S. Pat. No. 5,593,476.
Another attempt at improving filter system performance has been to place a first filter in an air flow path followed by a second filter media with spacing between them. It has even been proposed to electrostatically charge the second filter media when the second filter media has a greater air permeability than the first. Such an arrangement is shown in U.S. Pat. No. 5,871,567.
While the above advances provide improvements, those skilled in the art are always striving to develop better systems. This invention provides an enhanced filter arrangement with greater efficiency.
In general terms, this invention is a filter assembly for filtering out particles from an air flow. An assembly designed according to this invention includes a field generator that generates an electrostatic field. A filter is electrostatically charged by the field generator. The filter has an inlet side and an outlet side. The inlet side has a first porosity while the outlet side has a second porosity that is lower than the first porosity.
In one example, the filter has multiple layers of filter media between the inlet and outlet sides. Each of the layers is preferably electrostatically charged.
In one example, one of the filter layers serves as an electrode for the field generating device. In this example, the final layer at the outlet side of the filter preferably is a carbon impregnated foam that is capable of being charged and cooperating with another electrode to provide an electrostatic field across the filter material.
In another example, the filter is made using a single material that has an increasing porosity across the material. In one such example, a foam having a first density at the inlet side has a second, greater density at the outlet side.
The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiments. The drawings that accompany the detailed description can be briefly described as follows.